The invention relates to a series motor comprising contactless switching elements for interrupting operation, said elements being embodied in the form of electronic switches.
Power tools are generally driven by electric motors that are configured as series motors (universal motors) and controlled by means of mechanical switches. Although electronics are becoming more and more widespread in power tools as well, controlling power tools solely by means of power semiconductors that also serve to switch the power tool on and off is considered to be insufficiently reliable. The reason for this is that semiconductors can break down (internal short circuit) and then can no longer be controlled. In the case of purely electronic switches having only a single switch path, what can occur is that a permanent connection is created between the voltage supply and the motor, i.e. that the motor can no longer be switched off, or immediately starts to turn.
In order to achieve sufficiently reliable control of power tools using only electronic switches and no mechanical switches, it is known in the prior art to arrange a protective switch parallel to the control switch with which the motor is switched on and off, said protective switch tripping a fuse if there is any malfunction of the control switch (cf. DE 3 119 794 C2, DE 3 432 845 A1, DE 4 021 559 C1).
However, circuits of this kind operate reliably only when erratic behavior on the part of the machine and the electronic switching element is detected and the appliance can be brought to a defined OFF state, and any defect in the protection circuit can also be detected in order to put the appliance into a defined OFF state.
In the prior art, only DE 3 119 794 C2 addresses the problem of a defect occurring in the protection circuit, but said invention requires the operator to test the protection circuit occasionally by pressing a pushbutton.